Heteromorphic Sex Chromosomes in the Rare and Primitive Frog Leiopelma hamiltoni from New Zealand

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Abstract
Leiopeima hamittoni, one of the rarest and most primitive frogs, has a ZW♀/ZZ♂ chromosomal system of sex determination. The karyotype has 2n = 18 chromosomes, all of which are metacentric or submetacentric, except for the smallest pair, which are telocentric. The distribution of constitutive heterochromatin as detected by C-banding allowed all but two pairs of chromosomes to be readily identified. Two C-band heteromorphisms were observed. In all individuals of both sexes, a band was present on the long arm of only one of the homologous chromosomes no. 3. The other heteromorphism was sex-specific. The centromeric region of one of the telocentric chromosomes was enlarged and highly heterochromatic in females only and was interpreted as denoting the W chromosome. Female heterogamety may be a primitive condition in frogs.